Ohmic contact structures between semiconductors and electrodes are very important in realization of light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs), utilizing nitride-based compound semiconductors, for example gallium nitride (GaN) semiconductors emitting blue and green light and UV light. At present, commercially available gallium nitride-based light emitting devices are primarily formed on an insulating sapphire (Al2O3) substrate.
Meanwhile, these gallium nitride-based light emitting devices are broadly divided into top-emitting light emitting diodes (TLEDs) and flip-chip light emitting diodes (FCLEDs).
Currently used top-emitting light emitting diodes are configured so as to emit light through ohmic electrode layers in contact with p-type clad layers.
In addition, top-emitting light emitting devices can overcome problems associated with poor electrical properties such as low current injection and current spreading resulting from thin film characteristics of p-type clad layers having a low hole concentration, via development of ohmic contact electrodes having transparency and a low sheet resistance value.
In general, for these top-emitting light emitting devices, oxidized semi-transparent nickel (Ni)/gold (Au) metal thin films are widely used as metal thin film structures based on transition metals such as nickel (Ni) metal.
Nickel (Ni)-based metal thin films are reported to form semi-transparent ohmic contact layers having a specific contact resistance of about 10−3 to 10−4 Ωcm3 when annealed under oxygen (O2) atmosphere.
Such a low specific contact resistance of the ohmic contact layers, when annealed at a temperature of 500 to 600° C. under oxygen (O2) atmosphere, leads to formation of nickel oxide (NiO), a p-type semiconductor oxide, between gold layers formed into an island shape and on top parts thereof, at an interface between p-type gallium nitride and nickel (Ni), which results in decreased Schottky barrier height (SBH) and thereby easy supply of dominant carrier holes around the surface of the gallium nitride layer leading to increase in an effective carrier concentration therearound. On the other hand, it is understood that annealing of nickel (Ni)/gold (Au) after contacting with p-type gallium nitride removes Mg—H intermetallic complexes, thus leading to an effective carrier concentration at the surface of the p-type gallium nitride layer of more than 1019 via a reactivation process that increases a concentration of magnesium dopant on the surface of the gallium nitride layer, which in turn causes inversion of tunneling between p-type gallium nitride and the electrode layer (an oxidized nickel layer containing gold), thereby exhibiting ohmic conduction characteristics.
However, top-emitting light emitting diodes utilizing semi-transparent electrode thin films made up of nickel/gold have low light-utilization efficiency, thus making it difficult to realize high-capacity, high-brightness light emitting devices.
Recently, in order to realize high-capacity, high-brightness light emitting devices, there is a need to develop flip-chip light emitting devices using silver (Ag), silver oxide (Ag2O) or aluminum (Al) which are receiving a great deal of attention as materials for high reflective layers.
Meanwhile, such metal materials for reflective layers have high reflection efficiency and therefore can provide high transient luminous efficiency, but suffer from difficulty to form ohmic contacts having low resistance values due to low work function thereof, resulting in reduced life span of the device and poor adhesion to gallium nitride thus failing to provide stable device reliability.
More specifically reviewing problems associated with the use of silver and aluminum as materials for reflective layers:
Firstly, aluminum (Al) exhibits a low work function and easily forms nitrides (AlN) even at relatively low annealing temperatures, thus making it difficult to form ohmic contact with the p-type gallium nitride.
Next, silver (Ag) forms high quality ohmic contact and exhibits high reflectivity, but is heat-labile, thus suffering form difficulty to form high quality thin films via thin film forming processes. That is, silver (Ag) thin films exhibit agglomeration at the early stages of annealing due to heat-ability thereof and undergo changes into voids, hillocks and islands at the final stages of annealing, thus resulting in degradation of electrical and optical properties.
Recently, in order to extend applications of light emitting devices to high-brightness light emitting devices having large area and high capacity such as vehicle tail lights, domestic lighting and the like, extensive research into development of ohmic contact layers having a low specific contact resistance value while providing high reflectivity is being actively undertaken.
Mensz et al. have proposed nickel (Ni)/aluminum (Al) and nickel (Ni)/silver (Ag) structures as a bilayer structure (Electronics Letters 33 (24) pp. 2066), but these structures suffer from difficulty to form ohmic contacts and therefore raise problems associated with generation of large quantities of heat due to a high operation voltage upon operation of light emitting diodes.
Further, Michael R. Krames et al. have recently reported research and development of nickel (Ni)/silver (Ag) and gold (Au)/nickel oxide (NiOx)/aluminum (Al) electrode structures (US Patent Publication No. 2002/0171087 A1). However, these electrode structures also have shortcomings such as low adhesion and reduced luminous efficiency due to reflective scattering.